The idea of water boiling in a vacuum has to do with the air pressure, it lowers the boiling point of water. The outside air pressure exerts force on the surface of the water holding the molecules together with a bond. When the air pressure lowers the amount of energy(heat) required to separate those bonds lowers thus creating a lower boiling point. I would say in the vacuums used for glueing the pressure never drops low enough to actually boil the glue but it would help with the evaporation.

The results are a bit odd. Pretty much all the literature that I've come across suggests that the inclusion of a preservative in animal glues weakens it a little. This suggests otherwise but then again maybe Old Brown glue is bone rather than hide. It also doesn't state the gram strength of the hide. The other factor of course is the dilution of the dry hide which certainly does affect the outcome.Salt and urea are both hygroscopic, especially in high humidity environments. So they will perform well in normal environments but less so if kept in a warm, high humidity environment for long periods. I suspect we are getting into an abusive setting in terms of musical instruments though.

In the end though if you already have a glue pot and some hide glue it's cheaper to just make it yourself. I think someone here on this forum did a test once where they put a dab of HHG on a board next to a dab of OBG and waited. The HHG of course returned back to it's crystalline hard state but the OBG was rubbery for months. That test for all practical purposes could be meaningless though since the glue is not intended to be used that way obviously. But it's something to think about. On the other hand there are also people here who have built entire instruments out of OBG with no issues. One of my first guitars many years ago was built with Franklin LHG and after 25 years of abuse is still kicking. So there's that too.

Guess that's pretty good if you plan to sit on your guitar...More "group overthink" as far as I'm concerned. What will the difference be between 1500# and 1700# on a guitar. Also note that they used Elmers Carpentry Glue instead of Titebond regular. No cold creep. Glad I used HHG most all of those years...

Read the article while ago.Tnx david. I would take the results of this test with a grain of salt, pun intended. The problem is everyone has a different climate in which that glue is used. A fair comparison could be done with a mild dry climate vs a hot and humid one. e/g. I would use a fast drying glue like titebond 1. in a hot humid climate as michael said , using HHG could be problematic. Conversely when gibson was building their guitars in Kalamazoo michigan in the winter during the 1930/s they would heat up the glue room . so that the bracing could be put on using hhg, A cold room and using hhg would considerably shorten the open time for assembly. and create a multitude of gluing problems during cold weather. My conclusion , If I lived in hawaii, I would stick with either 195 . or 315 g HHG, and would add slightly less water to the glue. The warm and humid temps, would give a longer open time and no need for a depressant like salt or urea in the glue. as in the old brown glue formulation. The ancient egyptians used HHG in a hot and dry climate , examples 3500 yrs old using HHG are still intact.

One thing they didn't test for was resistance to heat. Hide glue does better than most to resist failing because of "hot car" conditions and is one reason many of us use it for highly stressed joints (bridges and braces).

I was the one that did the Old Brown Glue test leaving some in the bottom of a glue cup. Did the same with HHG, LMI White, and Titebond. The OBG never got hard. Stayed rubbery for months. The HHG dried to a glass like hardness. Had to chip it out of the cup. Titebond was pretty hard, but still would bend before breaking when out of the cup. LMI White (I miss that glue) was like glass too, and pretty clear. Recently I tried some HDX (Home Depot wood glue) - gets almost as hard and brittle as LMI white. I know it isn't how glues are supposed to be used, but if they don't harden in open air, will they ever fully harden in a joint? Most all of them will be stronger than the wood in a joint, but will they creep if they aren't.

Anyway, my OBG test made me toss the glue. I wouldn't use it on a guitar. But that's just me. I know others have been very happy with it. Might have been my batch. "Once burned, twice shy!"

I was the one that did the Old Brown Glue test leaving some in the bottom of a glue cup. Did the same with HHG, LMI White, and Titebond. The OBG never got hard. Stayed rubbery for months. The HHG dried to a glass like hardness. Had to chip it out of the cup. Titebond was pretty hard, but still would bend before breaking when out of the cup. LMI White (I miss that glue) was like glass too, and pretty clear. Recently I tried some HDX (Home Depot wood glue) - gets almost as hard and brittle as LMI white. I know it isn't how glues are supposed to be used, but if they don't harden in open air, will they ever fully harden in a joint? Most all of them will be stronger than the wood in a joint, but will they creep if they aren't.

Anyway, my OBG test made me toss the glue. I wouldn't use it on a guitar. But that's just me. I know others have been very happy with it. Might have been my batch. "Once burned, twice shy!"

How thick was the glue in the bottom of the glue cup? If you're reproducing what happens in a good glue joint it's a very thin, like a few microns, after it gets absorbed by the surrounding wood. I imagine the ammonia from the urea was trapped by the skin formed on the surface of the glue and not able to gas off.

How thick was the glue in the bottom of the glue cup? If you're reproducing what happens in a good glue joint it's a very thin, like a few microns, after it gets absorbed by the surrounding wood. I imagine the ammonia from the urea was trapped by the skin formed on the surface of the glue and not able to gas off.

Yep. It seems to dry ok in a joint, but if the gummy squeeze-out really bothers you, heat it and it will dry hard like HHG.

I use OBG for box closing. With HHG, it gels before I get the clamps on, and then starts to dry before I get to heating, and thus never fully squeezes out. I've never tried using OBG fully room temperature. It's pretty viscous, and doesn't fully squeeze out around the headblock until I heat it after the clamps are on. And the heat seems to break the waterproof film on the squeeze-out, so it dries like normal HHG.

I can confirm that the OBG is strong. Unbound white oak back glued in 35% RH is still fine after hanging on the wall in 75% last week (along with several others, but that one has the highest humidity expansion rate, so it's the most stressed of the bunch).

One thing that bears emphasis: it says on the bottles of Old Brown Glue that you should heat it before use. So, if you don’t heat it, you aren’t following the manufacturer’s recommendations. Like I said above, Old Brown Glue is just hot hide glue with a fair amount of urea added in. It is probably more convenient to try to use it exactly like you would aliphatic resin glue (ie, take the bottle of the shelf, open the spout and squeeze), and it might work OK for many things if used in that way. But it might not work for everything if used like that. It is going to work better for everything if you use it just like hot hide glue, with the exception that you don’t have to worry too much about gelling time, due to the presence of the urea. Since I make my own versions of Old Brown Glue and I’m committed to hot hide glue, that’s not a problem for me. But if you need a glue that can sit in a bottle at room temperature and be ready for use without any heating, you might be better off looking at fish glue or something else.

By the way, I did eventually find my notes on ratios. If you add urea that weighs 30% of the weight of the dry hide glue pellets, that will yield the equivalent of Old Brown Glue. Using urea that weighs 5-10% of the weight of the dry hide glue pellets will give a nice, limited extension of the gel time, but not really hurt the strength of the glue to any appreciable degree. I'm glad I found those notes!

Just for fun, I thought I'd do an experiment with my recipe for hot hide glue versus titebond.

I mixed 30 grams of 251 grade hide glue with 35 grams of water and 2 grams of urea (this doesn't make "room temperature" hide glue, just extends the work time a tad) ... and glued two struts to a spruce top off-cut, one with hide and one with regular Titebond. The clamping time was 1 hour.

After the hour was up ... I unclamped and tried to pull the struts off with my hands (didn't budge) ... then started tapping on the struts with my fretting hammer (didn't budge) ... then really started whacking at them with the hammer, to the point where I thought the spruce top would break ... both solid. I use the same drying time with hide glue as I do with Titebond when strutting a top (i.e. 1 hour for struts and 3 hours for pads).

During the glue up, I wiped a smear of excess glue on the spruce (at least 10 times thicker than a good glue line) ... both when I glued with Titebond and with the hide glue. I didn't think to pay attention to these smears of glue until the next day ... when I found the hide glue was hard as glass and the Titebond not too far behind (i.e. when I dragged my fingernail through the Titebond smear, there was a bit of drag ... not so on the hide)

This was all done in my climate controlled (40 percent RH at room temp) shop.

TNX , david you/ve only confirmed what I/ve always suspected. I stick with HHG for all my bracing, and if I/m in no hurry I use fish glue from ye olde cabinet shoppe in PA . or LV in canada. .I found in my testing , that it is much more difficult and takes a lot longer delaminate a joint with fish glue , which is why I use it to glue on tops and backs. Using HHG for me to do this task is a PITA, I also prefer fish for FB as it has a fast tack, and no need for slipping and sliding as with titebond and HHG.

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